Screen changers for use in polymer extrusion processes include a slide plate housed within a housing. The slide plate contains filter elements for filtering polymer and is translatable across an extrusion bore of an extruder apparatus. The surfaces of the slide plate contact and are sealed by a stationary downstream seal and a moveable upstream seal to prevent polymer from leaking from the extrusion bore between the slide plate and the housing. The upstream seal is generally biased against the slide plate surface to provide proper sealing conditions.
One current method of biasing the upstream seal against the slide plate is to thread the seal into the housing and rotate the seal with a wrench to adjust the seal axially into contact with the slide plate. A problem with this approach is that operator skill is required to ensure that the seal is tightened sufficiently to prevent polymer leakage but not so tight as to impede slide plate movement. Additionally, frequent readjustment is required to compensate for seal and slide plate wear.
Another method of biasing the upstream seal against the slide plate is disclosed in U.S. Pat. No. 3,962,092. Here the upstream seal is a free moving pressure actuated seal which is biased against the slide plate by the pressure of polymer within the extrusion bore. Such sealing systems have not provided consistent sealing.
Another method is disclosed in U.S. Pat. No. 4,359,387 in which the upstream seal is biased against the slide plate with a belleville spring. The disadvantage of this design is that the spring must exert a force great enough to ensure sealing under the worst-case design conditions such as pressures up to 10,000 psi. As a result, under most operating conditions the seal force is substantially greater than required, accelerating seal wear and necessitating additional power to move the slide plate.
Still another current method is to bias the upstream seal against the slide plate with an expandable polymer ring. The upstream seal has a conical surface at the upstream end which contacts a cooperating conical face on the ring. The ring is axially and radially confined by the seal and housing. The ring is made of a polymeric material with a coefficient of thermal expansion greater than the metal housing surrounding it such that, when the extruder is raised to operating temperature, the polymeric ring expands radially outward, wedging the upstream seal into initial contact with the slide plate. Further, under operating conditions polymer pressure acts on the internal surface of the ring, forcing the ring radially outward and wedging the upstream seal into tighter contact with the slide plate proportionate to the polymer pressure. The disadvantage of this design is that it relies on a polymeric material which is subject to thermal degradation and a decrease in properties at temperatures often encountered in extrusion processes. Generally, polymeric rings do not operate well at temperatures above 500.degree. F.